Code modulation communication system



Nov. y` 9, 1948.

J. c. scHs-:LLENG CODE MODULATION COMMUNICATION SYSTEM Filed June 19, 1946 2 Sheets-Sheet 1 A 7' TOR/VE V '.1. c. Ascl-naLLl-:NG

CODE MODULATION COMMUNICATION SYSTEM Nov. 9, 1948.

Filed June 19, 1946 2 Sheets-Sheet 2 j n G N R RNW m 0/ ma. A EH mm. Mx] 61M u y. B

Patented Nov. 9, 1948 rice l CODE MODULATION COMMUNICATION SYSTEM John C. Schelleng, Interlaken, N. J., assignor to Bell Telephone Laboratories, Incorporated,

New York, N. Y., `a corporation of NewYork Application June'IQ, 1946, Serial No. 677,667

This invention relates to communication systems for the transmission of complex wave forms of the type encountered in speech, music, telegraph, facsimile and televisionsignals.

Communication systems have been proposed in which the instantaneous amplitude of a complex wave to be transmitted is vrepresented in binary is claims.v (C1. 17e-1.5)

` tions is not required, and in which the complexityV code form for transmission to a receiving station. In such systems, the amplitude of thecomplex wave in arbitrary units may be expressed as or represented by a binary number and currents or voltages which may be turned on or oif in different circuits are assigned to represent each digit, i. e., denominational order, of that number. Thus each digit voltage or current may by its presence indicate a 1 and by its absence a 0 in the corresponding or representative binary numbering system. In certain prior communication systems of this type, the amplitude of the complex wave to be transmitted Vis sampled periodically and the sample amplitudes are converted into a binary code group of voltages. `These code groups are transmitted to a receiving station at which equipment, carefully synchronized with that at the transmitter, is utilizedvto reconstruct the original complex wave. v

This method of transmission has certain inherent advanta-ges. Among these the most im' portant arise from the fact that the transmitted signal comprises either a voltage or no voltage or at least one or the other of two signalling conditions for each digital position ordenominational order of the code group. Consequently the receiver need only be capable of distinguishing between the two signalling conditions to permit faithful transmission of the complex Wave. Nonlinearity of the transmission link and vother amplitude distortion can be tolerated.` In addition a low signal-to-noise ratio can. be accepted in the transmission path so long as the presence of the voltage representing the ls of therbinary code can be distinguished from the noise.` i

Systems of the type considered above,Y however, involve provision of accurate means for synchronizing the operation of a number of widely separated units, the operation of which `must be timed with high precision. Furthermore, they require the use of separate sampling and coding systems and relatively complex auxiliary timing devices at the transmitter to insure their sucinvolved in first sampling and then coding in separate circuits is eliminated. Itis a further object of the inventiontc achieve such simplification without sacrice -of the acivantageous results outlined `above in connection with the prior art systems. There is provided in accordance with one aspect of the invention a communication system in which the amplitude of the complex waveto be transmitted Ais directlyv and continuously convert-v ed into a binary code group representation which is then transmitted to a receiving station at which the elements of the code group are combined to reconstructtheoriginal complex wave.

In other aspects the invention relates to -a binary coder Which is operated continuously to produce a binary code group which at all times represents the instantaneous amplitude ofthe complex wave to be transmitted and to `receiving circuits for translating a code group into the complex Wave amplitude represented thereby.`

' The communication system of the invention'is described and shown herein as arranged Y. for the transmission of voice frequency intelligence but it is to be understood that this system may be used equally well for the transmission of tele-V graph, picture or television signals through modi'- iication of the terminal equipment utilized' at the transmitter preceding the coding equipment and at the receiver following the decoding equipment. j The above and other features of the invention will be understood by reference to the accom` panying drawings in which: Fig. 1 is a block diagram of a communication system in accordance with the invention; and" Fig. 2 is a schematic diagram partially inblock form of the binary coder utilized at the transnfiit` ting station of this communication system.

Referring to Fig. 1, the communication system of the invention is shown as comprising a transmitting station, a transmitting link `and a receiving station. yAt the transmitting station complex wave voice-frequencyA currents from microphone l0 are applied to terminal equipment i2, which may comprise the necessary switching and transmitting devices including amplifiers and impedance matching devices to convey the complex wave form from source l0 to the coding and transmission system described herein. Equipment I2 is also employed to add a bias tothe complex wave if-necessary to prevent it from having negative va1ues...As a result,f the output signal from equipment I2 is always of positive polarity.

The signal from the terminal equipment, which will be referred to herein as the input signal is applied to a series of selectors I4 through 24 comprising over-biased and limited amplifiers or amplitude,v selective repeaters, each of which is responsive to a different fixed portion of the applied signal. Each of these amplitude selectors or relays I4 through 24 is biased so that it is responsive only to input voltages. larger than` a. predetermined designated input voltage or threshold and is operative to produce zero output voltage for all input voltages less than the designated input voltage, andan output voltage equal to the designated input voltage for all inputs equal to or greater than. the designated input voltage. As sho-wn in Fig. l, the input signal is applied directly to selector I4, while the' diierence between the input signal and the output: oft selector I4 isv applied' to a secondselector I6. Input connections similar to those to selector I6 are made to selectors I8', 2U', Z2 andZ, the signal'l applied to each of these'selecti've switching devices being the difference between the input signal andi the output of the preceding selective switches.

Irr the system shownI for purposes of' illustration'A in Fig, 1, the'designated input voltages of selectors' I4 through 24' are'related as descendingY powers of' two', selector It beingY responsive onlyA izo-input' voltages greater than 32: volts (two to' theffth power). Thus, selectors I6, I8, 2D, 22 and 2&1 have designated inputfvoltages ofv 16, 8, 4, 2 and 1 Volt, respectively. RecallingA that the output of each selector must be' either zero or its designated input voltage, itwill be seen that selector I4-l may have ank output potential of either zero or 32'volts while'selector I6 may'produc'eza'potential diierence'at its output: of` either zero orf16 volts: Similarly', selector I8A mayv produce av potential difference' of' either zero' or 8 volts.` Since-'the remaining: selectors operatefin the same manner, it will. be seen'. that any' input voltagezfrom to'63,volts, inclusive, may beexpressedy as the sum of. the potentialA diierences ther preceding selector in thev chain.. Accord-- ingly, selectors I6l through 24 areprovided with auxiliary devices 2.6. through 36, respectively whichwillV be` referred to herein as digit grounders. Each of these devices isv arranged. to convert the potential difference existing at the output of the selector with which it. is associated into a potential in respect tov ground. In one embodiment of the invention the potentials in respect` to ground at the outputs of the several digit' grounders are positive when the associated selectors are operative and are zero at all other times. Thus' there is continuously,l pro*- Vi'dedtlon' output terminals 38 throughA abinary coderepresentation of theI varying# amplitude of the complex wave introduced through terminal equipment I2. It will4 be understoodv that the signal appearing at'v any one'. of these output terminals is a voltage corresponding to one of two signalling conditions, this voltage appearing and disappearing as necessary to express the instantaneous amplitude of the complex wave to be transmitted.

The six output signals are transmitted over separate` paths or channels, which may be wire transmissionlinks -or radio links operating at diiferent carrier frequencies, to a receiving station shown at the right in Fig. l. At the receiving station,` they signals from the six transmission paths or channels are applied to separate directcurrent amplifiers and limiters all through 6B. In theillustrative system, each of these identical amplifier-limiter combinations is arranged to produce an output of 32 volts for any input signalexceeding a lower limit which is made high enough to exclude extraneous transmission noise', andto produce Zero output for inputs below this limit. Thus amplitude distortion or attenuation introduced by the transmission link donot affect the operation of the receiver unless the signals in one or more of the transmission paths are so reduced in amplitude that they do not exceed thevnoise-level threshold introduced by the-limiter.

The output of ampliier-limiter 5u which is 32 volts or zero, depending upon whether or not selector lli-at the transmitter is operated, is applied through series resistor 6?. to resistor 63 which has'a` low ohmic value in relation to that o resistor 62. The:l output of amplifier-limiter 52, however, which is also either 32 volts or zero, depending upon whether or not selector I6 at the transmitter is operated, is applied to an att'enuator 64 arranged to provide an output equal to one-half of the Voltage applied thereto or, 16 volts. This output voltage is applied through resistorv 66, which has the same resistance as resistor '62, to resistor 63. In similar fashion attenuators 63, 12,16 and are associated with amplifier-limiters 56, 56, 53 and 69, respectively and provide. output voltages of 8 volts, l volts, 2'volts or 1 Volt which are respectively applied n to'resistor 63 through resistors lll, lll, "i8 and B2 each equal in ohmic value to resistor 62, when signals are present in the corresponding transmission channels. The resistive network comprising series resistors 62, 66, 10, Iii, it and. 82 and' resistor 63 is such that the outputs of the six receiving channels are eieotively combined for application to receiving terminal equipment 84", which comprises the amplifiers and other audio-frequency circuits necessary to provide a suitable-input for headphones 86.

In the operation of the system described above, the complex wave appearing at the output of terminal equipment I2 is translated into a 6element binary code, the digital positions or denominational orders of which are represented by output terminals 38 through 66, voltages appearing at each of.V these terminals as required to produce the proper'binary code representation of theinstantaneous amplitude of the complex wave to be transmitted. The voltages existing at terminals 38 through I8 are transmitted to the receiver at which there is derived from each actuated channel a Voltage having an amplitude eoua-l to that to which the selector associated with the same channel is responsive. These voltages are then continuously added to reconstruct the complex wave applied to the transmitter.

Details of the selectors and digit grounders and the methodsof Y interconnection between succes- `bias battery 98 to the plate of diode 04.

sive selectors in the chain are shown in Fig. 2. With the exception of operating potentials, the six selectors are similar and a detailed description of selector I4 will permit a` complete understanding of all of the selectors. The output of terminal equipment I2 is applied directly to this selector which as described above has a designated input of 32 volts and is arranged to provide an output of either zero or 32 volts, depending upon whether the input voltage applied to it is less than 32 volts or is equal to or larger than 32 volts. The input voltage is applied first through a series resistor 96 to a biased diode 04, battery 98` serving to maintain a negative bias of 32 volts on the plate of diode 04. If the inputvoltage exceeds. 32 voltsdiode 94 becomes conductive and a voltage drop is developed across resistor |00 connected between the cathode of diode 04 and ground.

The voltage appearing across cathode resistor |00 is applied to a two-stage direct-'current amplifier comprising triode-type vacuum tubes |02 and `|04 with their associated circuit elements and sources of potential. This amplifier serves toraise the power level of the signal appearing *across resistor |00. The cathode of` triode |02 is connected to ground, while the plate circuit includes a battery |06 and a series load resistor |08. The potential drop occurring across load resistor |08 is applied to the grid of triode |04, the cathode of which is grounded and the plate of which is connected through load resistor IIO to a tapped battery II2, the negative terminal of which is grounded. The output of this directcurrent amplier is Aapplied to a limiter circuit comprising diodes ||4 and IIS.` The cathode of `diode IIA is connected to the plate oi triode |04,

while the plate of diode |I4 is connected to a tap on battery II2 such that the applied plate potential is 50 volts. The plate of diode I I0 is connected to the plate of triode |04, while its cathode is connected to a tap on battery I|2 such that the potential applied thereto is equal to 50+32 or 82 volts.V A 50-volt bias battery IIB is connected between the plate of triode |04 and output terminal |20 and serves toY oppose the potential appearing at the plate of triode I|4, making the potential at terminal |20 equal to the `potential at the plate of triode |04 less 50 volts.

In considering the operation of this selector, let it first be assumed that an input signal greater than 32 volts is applied through resistor 96 and Because the negative bias on this diode is exceeded, it will become conductive and a voltage drop will be developed across resistor |00. This voltage drop raises the potential applied to the control grid of triode |02, thus increasing the plate current of that tube. Such an increase in plate current increases the voltage drop across load resistor |08 lowering plate voltage of triode |02 and decreasing the potential applied tothe control grid of triode |04. The decrease of plate current through triode |04 caused by the decrease of the applied grid potential results in an ,After the 50 volts provided bybias battery IIB is subtracted from the voltage appearing at the voltage of triode |04 falls below 50 volts.

plate of triode |04, the output voltage produced at terminal |20 is maintained at 32 volts so long as the input signal is equal or greater than 32 volts.

Let it now be assumed that the voltage applied to diode 94 is less than 32 volts. Under this condition the negative bias of battery 63 is not exceeded and diode 04 does not conduct. Accordingly, no potential drop occurs across cathode resistor |00 and the plate current through triode |02 assumes a value for which the voltage drop through resistor I 03 is considerably less than that occurring when diode 04 is conducting. The Voltage across the plate resistor |08 of triode |02 which is applied to the grid of triode |04 is thus higher, i. e., less negative and the last-mentioned tube draws a plate current which is considerably greater than that drawn when diode 9d is conducting. Under these conditions, a relatively high voltage drop occurs across load resistor ||0 tending to lower the voltage at the plate of triode |04. This voltage is prevented from falling substantially below 50 volts, however, by limiterdiode IIA which has a positive plate bias of 50 volts and becomes conductive when the plate When this diode becomes conductive, a portion of the plate current drawn by triode itil ows from battery ||2 through diode H4 rather than through resistor ||0 thus holding the plate potential of triode |64 at 50 volts. When the negative bias of 50 volts provided by battery H8 is subtracted from the plate potential of triode |04, it will be seen that under the input signal condition last assumed, the potential appearing at output terminal |20 is zero. Thus it will be understood that selector or relay I4 will provide an output voltage of substantially zero yfor all input voltages less than 32 volts and an output voltage of substantially 32 volts for all input voltages equal to or greater than 32 volts.

The potential appearing at output terminal |20 of selector I|4 is applied to transmitter output terminal 38 and to the input of a second selector I6 to which is also applied the output of terminal equipment I2. It will be seen from Fig. 2 that selector I6 is similar to selector i4 with the eX- ceptions that the cathode resistor of diode I 22, the cathode of direct-current amplifier triodes |24 and |26, and the negative terminal of tapped battery |28 are connected to output terminal |20 of selector I 4 rather than to ground. In addition, bias battery |30 provides a negative bias of 16 volts, while the tap to which the cathode of limiter-diode |32 is connected on battery |28 is chosen to provide an applied voltage of 564-16, or 66 volts.

Thus there is applied to selector 16 the output of terminal equipment I2 `less the output of selector I4 whether the output of selector I4 happens to be zero or 32 volts. The operation of selector I6 is identical to that of selector I with the following exception produced by the changes in battery potentials and input connections described above. If the difference between. the instantaneous amplitude of the complex wave appearing at the output of terminal equipment I2 and the potential appearing at the output ter- `minal |20 of selector I0 is equal to or greater than 16 volts, a potential diference of 16 volts will be produced between output terminal |34 of selector I6 and output terminal |20 of selector I4. If, on the other hand, the diierenoe between the output of terminal equipment I2 and that of selector I4 is less than 16 volts, zero potential grid-cathode drop of pentode selector terminals ,|2l and |34 into a potential in respect to ground which appears at transmitter output terminal 40.

Digit grounder 2S in the exemplary embodiment `described in detail herein v.comprises a pair of pentode-type vacuum tubes |36 ,and |38 which are connected for operation as cathode followers. Thus, the potential appearing at terminal |34, the output terminal of selector I6, is applied to the control grid of pentode |36, the cathode of which is connected through resistor M to ground. Battery |42 is connected in series between the `plate of pentode |36 and ground to apply positive potential to the plate. The screen grid of pentode |36 is connected to the positive terminal of battery M6, the negative terminal of which is connected to the cathode, while the suppressor grid is connected directly to the cathode. The cathode of pentode |36 is also connected through resistor |138 to terminal |5il.

The output of selector III appearing at output terminal is applied to the control grid of pentode |38, the cathode of which is connected through resistor |52 to ground, while plate potential is provided by battery |54 the positive terminal of which is connected to the plate ,of pentode |38 and the negative terminal of which is connected through resistor |56 to ground. The screen grid of this vtube is connected through battery .its to the cathode, while the suppressor grid is connected directly to the cathode. The negative terminal of battery |54 is also connected through a resistor |62 to terminal |50.

Considering the operation of the digit grounder described above, it will be seen that the output `voltage of selector i6 applied to the control grid of pentode |36 causes the `production of a voltage drop across cathode resistor |40 substantially equal to the voltage at terminal |34, less the |36. Similarly, ythe output at terminal |20 of selector Ill is applied to the control grid of pentode |38 and produces a voltage drop across cathode-resistor |52 substantially equal to the potential at terminal IZil less the gridcathode drop of pentode |38.

. Cathode resistor |52 of cathode-follower ypentode |38 and series resistor |55 in the platecircuit thereof are so proportioned that the voltage developed across cathode resistor |52 for a given plate current is reproduced across resistor |56 with opposite polarity. Thus, a voltage proportional to that appearing at terminal |34 of selector It is applied through resistor |48 to point |50, While a voltage proportional to the inverse of that appearing at terminal |20 of selector i4 is applied through resistor |62 to the same point.

.Resistors Itii and |62 have equal high ohmic values of the order of megohms and the voltage across cathode resistor I d0 of pentode |35 and that across plate resistor |56 of pentode |38 are -averaged at terminal |50. Thus it -will be seen that the voltage appearing at terminal |50 rand applied to transmitter output terminal all is either ,zero or eight in respect to ground, (a voltage equal to one-half lthe potential difference produced by coder I5) depending upon whether or not the selector receives an input signal less than or .equal to or greater than I6 volts.

The input connections to the remaining selectors I3, 20, 22 and 24 are similar to those to selector I6 and the digit grounders 28, 30, 32 and 36 are similar to digit grounder 26. In the exemplary system described in detail herein, these 'digit `grounders provide output voltages in respect to ground of 4, 2, 1 and 1/2 volts, respectively, when the associated selectors are actuated and zero output at all other times.

Many of the circuits utilized at the receiving station and indicated in Fig. 1 may be of wellknown types and are not described in detail herein.l However, the direct-current amplifier-limiter combinations 5t through 60 may conveniently be similar to those utilized in selector I4 with the exception of changes in certain of the applied battery potentials. Thus, referring to the showing of selector lll in Fig. 2, the potential of the selector bias battery (98 in Fig. 2) is made slightly greater than the largest noise voltage to be tolerated in the transmission link. Then, any signal voltage exceeding the noise level will cause the diode (9i) to conduct, providing an increase in the voltage app-lied to the direct-current arnplier. This amplifier and the limiter may be identical in construction and operation to those employed in selector I4 and the unit as utilized at the receiver provides an output of 32 volts whenever the noise level is exceeded and zero output at other times. It will be understood that equivalent direct-current amplifiers and limiters may be substituted for those described above, both in the selectors and in the receiver.

Attenuators 61|, 66, 12, 'I6 and 80 may comprise resistors connected across the outputs of the corresponding arnplier-limiters and tapped at appropriate points to produce attenuations of 1/2, 1/4, 1/8, Tg and gli corresponding to output voltages of 16, 8, 4, 2 and 1 volts, respectively. Other resistive attenuating networks of more complexity may be used if desired.

The system as described above is susceptible of modification and considerable simplification if high fidelity transmission links are available. Thus, if the six channels of the transmission link are free of amplitude distortion and introduce either zero or xed and equal attenuations, the amplifier-limiter equipment may be eliminated from the receiver of Fig. 1. Under these conditions the signals transmitted from the digit grounders associated with the l, 2, 4, 8 and 16-volt selectors may be applied directly to the additive network comprising resistors 62, B3, 66, 10, 14, 18 and 82 of the receiver of Fig. 1. Since the digit grounders have outputs of either zero or one-half the designated voltages, depending upon whether the associated selectors are actuated, while selector Ill has an output of either zero or the designated voltage, means must be provided for reducing by one-half the signal applied to the additive network at the receiver by selector I4 at This may be accomplished in several Ways. Forv example, a potentiometer or other attenuator may be used either at the transmitter or at thereceiver to halve the signal in the channel controlled by coder I4.

As will be realized, the system as described above and shown in the drawings may be subincrease the range of amplitude accommodated.

The addition of selectors atthe transmitting station requires the provisionA of an equal number of transmission channels in the transmission link and also the 'addition` o `further direct-current -amplier-limiters and attenuators of appropriate value atthe receiving station.Y

What is -claimed is: l Y i n i 1. Ina communication system for transmitting complex waves, a plurality of selectors, each responsive to a dierent'iixed portion of thetotal possible magnitudeof the wave to betransmitted, j

each of said selectors having Zero Aoutput for inputs less .than the portion of the total magnitude to which it is responsive, and an Aoutputrequal` to that portionfor all other inputs, means for applying the waveto `be.transmitted to the` selector v:

responsive to .the largest of` said portions, Vand means for applying to each of the other selectors the diiTerence between the instantaneous magnitude of the wave to betransmitted, yand theoutput of the selector-resp-onsive to the nextxlarger .al

portion of saidmagnitude.

2. In a communication system for transmitting complex waves, `a plurality of selective relays, each responsive toa different iixed portion of thetotal possible magnitude ofthe complex wave, each of said relays having zero `output `for inputslless than the portion of the total possible magnitude to which it is responsive and being capable of producing a difference in 'potential equal' in magnitude to that portion for all other inputs, means for applying the wave to `be transmitted tothe relay responsive to the largestportion of the total possible magnitude, and means `for applying to each of the other relays a potential equalto the magnitude of the wave to be transmitted less the magnitude of the potential diierence at theoutput of the relay responsive to the next larger portion of theltotal possible magnitude. i

3. In a communication system for transmitting complex waves, a `plurality of repeaters, each responsive to a different fixed portion of: the total possible magnitude of the complex wave, `each of `said repeaters having zerooutput for `inputs less than the portion of the total possible magnitude to which it is responsive -and being capable of producing a; difference in` potential equal in magnitude to that portion of thetotal possible magnitude for al1 other inputs, means for applying the wave to be transmitted to the repeater responsive to the largest fraction of the total possi.. ble magnitude, means for applying to each of the other repeaters a potential difference equal to the magnitude of the wave to be` transmittedless the magnitude` oi the potential difference at the output of the repeater responsive to vthe next larger portion of the total possible magnitude, and means for converting the diiierences in potential at the outputs of said repeaters into potentials in respect to ground. Y l I. f 4. In a communication system for transmitting complex waves, a plurality of selectors, each re'- sponsiveto a different portion of the total possible magnitude of the complex wave, each of said selectors having zero output for input voltages less than the portion of the total possible magnitude to which it is responsive and being capable of` producing a potential in respect to ground equal in magnitude to that portion of the complex wave for all other inputs, means for applying the complex wave to be transmitted to the selector responsive to the largest portion of the total possible magnitude, means for applying to eachof the other selectors, the difference between themagnitude of the wave t0 be transmitted and the output of the selector responsive to the next larger portion ofA the total, and means for transmitting the output potentials of the selectors to a remote station.

5. A communication system for transmitting complex waves, comprising a plurality of selectors, each responsive to a diierent fixed portion 'of the total possible magnitude of the complex wave, each of said selectors having zero output for inputs less than the magnitude of the portion of the total possible magnitude to which it is responsive and being capable of producing an output proportional to the magnitude of that portion for all other inputs, means for applying the complex wave to be transmitted to the selector responsive to the largest portion of the total possible magnitude, means for applying to each of the other selectors` the difference between the waveto be transmitted and the output oi the selector responsive to the next larger portion of the total, means responsive to the outputs of the individual selectors for producing equal potentialsywith respect to ground, means for transmitting these potentials to a receiving station, means thereat for converting said last-mentioned potentials into potentials proportional to the output of the selectors associated with each, and means for adding `the potentials so derived together.

- 6; In a system for expressing the magnitude of a complex wave in terms of aplurality of fixed voltages related as powers of two each of which represents a different portion of the total possible amplitude of said complex wave, a plurality of selectors each controlling one of said fixed voltages and each being responsive only to input voltages at least as great as its output voltage, means for applying a complex wave to the selector responsive tothe largest input voltage, and means for applying to` each of the other selectors the voltage differences between the magnitude of the complex lwave and the output of all selectors responsive to larger inputs. e

- .'7. In a system for generating binary code representations of the magnitude of a message wave, a group of selectors eachresponsive only to an input greater than its individual operating `input, the operating inputs of said selectors being related in magnitude as powers of two,v means for applying the message wave to the selector having the largest operating input, and means for applying to each of the other selectors an input signal equal to the magnitude of the message wave less the operating inputs of all operated selectors having larger operating inputs.

8. vIn a system for generating binary code representations of the magnitude of a complex wave, a group of selectors each responsive only to an input greater than a predetermined magnitude, the predetermined magnitudes of said selectors being related in magnitude as powers of two, means for applying the complex wave to the selector gespeiresponsive to the largest 'predetermined magnitude, other means for applying to another one of said other selectors an input signal equal to the magnitude oi the complex wave less a function of the output of a selector responsive to the largest predetermined magnitude and means for producing output signals for the selectors.

9. In a system for generating binary code representations of the magnitude of a complex wave, a group of selectors each responding only to an input greater than its individual threshold, the thresholds 'of said selectors being related in magnitude as powers of two, means for applying the complex Wave to the selector having the largest threshold, means for applying to each of the other'selecto'rs an input signal equal to the magnitude of the complex Wave less the sum of the thresholds of all responding selectors having larger thresholds and means for producing outp'ut'signals for the selectors.

10. In a system for generating binary code rep-y resentations o the magnitude of a complex wave, a group of selectors each responsive only to an input greater than its individual threshold, the thresholds of said selectors Ybeing related in magnitude as powers of two, means for applying the complex wave to the selector having the largest threshold, means for applying to each of the other selectors an input signal equal to the magnitude of the complex wave less the sum of the thresholds of all responding selectors having larger thresholds, means for producing output signals for the selectors, and separate channels for transmitting said signals to a receiving station.

l1. In a communication system for transmitting complex waves,- a coder for generating binary code representations of the instantaneous amplitude of a complex wave to be transmitted, said coder comprising a plurality of selectors. each controlling an output signal representative of a different fixed portion of the total possible amplitude of said complex wave, means for applying the complex wave to all of said selectors and means for applying in opposition to said wave to each of said selectors, excepting that controlling the signal representing the largest portion of said total possible amplitude, voltages proportional to the sum of the amplitude portions represented by the outputs of all selectors corresponding to larger portions of said total possible amplitude.

1'2. In a system for generating binary code representations of the magnitude of a complex wave, a group of selectors each responsive only to an input greater than its individual operating input, the operating inputs of said selectors being `related in magnitude as powers of two, means for applying'the complex wave to the selectorhaving the largest operating input, means for applying to each of the other selectors an input signal equal to the magnitude of the complex wave less the sum of the operating inputs of operated selectors having larger operating inputs, means 'for producing output signals for the selectors which are operated, separate channels for transmitting said signals to a receiving station, means at the receiving station for converting the transmitted operating voltages into voltages proportional to the operating voltages of the selectors represented thereby, and means for adding the converted voltages together.

13. In a communication system for transmission of complex waves in which a signal is generated as a difference in potential between two ungrounded points, means for converting said 1.2 difference fin-'potential 'into a :potential 'with 'respect to ground-comprising a converter for transforming the potential a't one 'of said pointsinto a potential `With respect to ground, a second converter fortransform'ing the potential at the other of -said points `into a potential with respect to ground, means for inverting the output of the second converter .and means Ifor averaging the output of theiir'st converter andthe inverted output yof `the secondconverter.

'14. Ina communication Asystem for transmission of complex wave'szin which a signal is generated as a difference in 'potential Ibetween two ungrounded points, means for converting said difference in potential into a vpotential with respect to ground comprising a pair of cathode 'follower tubes e'a'ch Vhaving lat least a cathode, an anode and 'a control element, the cathodes b'eingigrounded ``through load resistors and the anodes being connected to sources-of positivepotential, means for applying said difference in `potential between said control elements, means for inverting the cathode potential of one of said cathode followers and means for averaging said inverted potential with the 'cathode potential of the other of said cathode followers.

15. In a communication system for transmitting 'complex Waves, a vplurality of signaling means each of which may have either of two output conditions one of which in each case represents a different/component of the total possible amplitude 'range of said complex wave, means responsiv'e to said wave for deriving therefrom a plurality "of control `quantities equal in number to said amplitude components `and continuously indicative of the presence :or'absence of the corresponding 'components in the wave to be transmitted, and means for applying said quantities at all 'times .to control the output conditions of the corresponding signaling means.

l6. In a communication system for transmitting complex waves, '-a plurality of signaling meanseach of which may have either of two output' conditions lone 'of which in each case represe'nts a y'different 'component of thetotal possible amplitude lrange 'of said complex wave, and the combinations of which represent all amplitudes in said `'total l.possible range in integral steps determined by the smallest component represented in vany case, means Iresponsive to said Wave for deriving therefrom a plurality of control quantities continuously .indicative of the presence or absence of the corresponding components in the wave to be transmitted, and means for applying sai'dnuan'tities at all times to control the output conditions lof the corresponding signaling means. Y

1/7. -In a communication system for transmitting complex waves, a plurality of signaling means each of which may have either of two output conditions one of which in each case represents a different component of the total possible amplitude :range of said wave and the combinations ofwhich represent all amplitudes in said range in integral steps determined by the smallest componentfrepresente'd 'in any case, and means for 'applying said wave 4to said signaling means to continuously rcontrol the output conditions of said signaling -means in accordance with the presence or absence ofthe corresponding amplitude components in said wave.

18, In a communication system 'for transmitting complex waves, a plurality of selectors each responsive to a different fixed component of the totalpossible amplitude of the wave to be transmitted, each of said selectors having zero output UNIT for inputs less than the `component of the total ED STATES PATENTS amplitude to which it is responsive and the same Number Name Date plying the Wave to be transmitted to the selector 5 2,282,046 GOldSmith May 5, 1942 means for applying said Wave to each of the other 2,403,210 Butement July 2, 1946 selectors eiectively subtracting from said Wave 2,430,139 Peterson NOV. 4, 1947 quantities proportional to the components repre- FOREIGN PATENTS sented by each other selector which has responded l0 to a larger component of said wave. Number Country Date JOHN Q SCHELLENG, 047,468 Germany -c July 5, 1937 REFERENCES CITED The following references are of record in the 15 le of this patent: 

